The photoinduced polymerization of monomers or oligomers comprising functional groups prone to radical reactions is a widely used technique in the production of polymers. To start the reaction usually photoinitiators are added to the monomers or oligomers and the mixture is then exposed to electromagnetic radiation. Many photoinitiators are available which are readily soluble in unsaturated organic resins, and effectively cure these. However, many of these photoinitiators have poor solubility, and therefore unsatisfactory curing effectiveness, in silicones. Such photoinitiators separate from the silicone matrix during storage causing severe haze in any cured product produced. This is not acceptable if the products are intended to be used for optical clear display applications. In order to increase solubility in and hence compatibility with silicones it has been proposed to chemically bond the photoinitiators to organopolysiloxanes or silanes.
U.S. Pat. No. 4,273,907 discloses a novel class of organopolysiloxane compounds comprising at least one benzoin group chemically bonded to a silicon atom of the organopolysiloxane molecule. Such compounds may be prepared by the dehydrohalogenation, dehydrogenation, dehydration or dealkoholation condensation reaction between a corresponding organopolysiloxane having silicon-bonded halogen atoms, hydrogen atoms, hydroxyl groups or alkoxy groups and a benzoin compound in the presence of a suitable reaction promoter or a condensation catalyst. They are soluble in silicone compositions and useful as a photosensitizer in photocurable organopolysiloxane compositions. However, the compounds do not show sufficient stability and tend to cause yellowing upon exposure to heat and/or UV radiation over a longer period. Hence, they are not suitable to be used in high-performance transparent coatings, encapsulants or sealants.
U.S. Pat. No. 4,391,963 also discloses novel photosensitizers, being organopolysiloxane compounds bearing at least one chemically bonded benzoin group. Here, however, the compounds are prepared by hydrosilylation of an alkenyl-substituted benzoin and a silicon compound containing at least one silicon-bonded hydrogen atom. Hence, the organopolysiloxane moiety and the benzoin group are bonded via a divalent hydrocarbon group. This results in an increased stability against hydrolysis, however, requires a comparatively elaborate preparation process since alkenyl-substituted benzoins are not readily available, but have to be synthesized in a separate reaction step.
From U.S. Pat. No. 4,536,265 are known further organopolysiloxane photoinitiators. At least one siloxane unit per molecule comprises an acetophenone photomoiety bonded to the silicon atom of the siloxane unit via a divalent hydrocarbon group having between 2 and 10 carbon atoms. Such a binding group shows high stability against hydrolysis. However, again alkenyl-substituted acetophenones are not readily available, but have to be synthesized in a separate reaction step, making the overall process to produce the organopolysiloxane photoinitiators complex. Moreover, acetophenone moieties tend to cause yellowing upon exposure to heat and/or UV radiation over a longer period.
U.S. Pat. No. 5,776,658 describes silicone-compatible photoinitiators comprising a silane or organopolysiloxane residue bearing a chemically bonded radical of a compound which has one or more of a photoinitiator or photosensitizer activity and which has at least one carbonyl group located on an aromatic nucleus. Again, the bonding of the silane or organopolysiloxane residue and the photomoiety is effected via a divalent hydrocarbon group. This divalent hydrocarbon group is attached to an aromatic carbon atom which is positioned ortho to the carbonyl group of the photomoiety. The photoinitiators show good stability against hydrolysis. The examples disclose preparation of the photoinitiators via a one-pot reaction. However, the resulting product mixture needs to be concentrated and worked up by chromatography. So the overall process is complex and time-consuming. Moreover, the yields of photoinitiator are rather low, e.g. only 17% in example 5.
EP 1072326 A2 provides siloxane-containing surface-active photoinitiators which concentrate on the surface of the formulation. These photoinitiators are used in a method for producing scratch-resistant coatings from formulations containing ethylenic compounds. The photomoiety comprises an aromatic ring which is linked via a bridging unit Y to a silicon atom of the siloxane residue. Due to the surface-active properties of these photoinitiators they will not be homogeneously distributed throughout the formulation and, hence, are not suitable to be used in high-performance transparent coatings, encapsulants or sealants.
It is evident from the above documents that although many photoinitiators have been developed which may be used in the curing of silicone resins there is still need for improvement.